The present invention relates to multilayer printed circuit boards, and more specifically to a transition between symmetric and asymmetric striplines in such boards.
Many different sorts of multilayer printed circuit boards are known to the art. LTCC (Low Temperature Co-fired Ceramic) will be used hereinafter as an example, although it will be understood that the invention can also be applied in other types of multilayer printed circuit boards.
Briefly, multilayer printed circuit boards are manufactured in the following way. There is obtained on the basis of a printed circuit board design a drawing that contains necessary information, such as the number of layers, the appearance and dimensions of the patterns on the various layers, the locations at which different layers shall contact one another, and so on.
Each layer per se is rolled out from a ceramic mass to a predetermined thickness on a plastic film; this is a so-called tape. Different patterns are punched from these tapes in accordance with the design; among other things, the outer edges of the board, the marks that are later used to match the layers together, and holes for binding different layers together with so-called vias.
Subsequent to configuring the layers, the via holes are filled with a suitable conductive material. The patterns are then printed on each of the layers. A common method in this respect is to use screen printing to correctly position the conductors. These conductors may consist of gold, silver or some other suitable conductive material. When the patterns are in place, the various layers are placed one upon the other until all layers are in position.
The whole of the printed circuit board is then placed under pressure, inserted into an oven and baked immediately (Co-fired) at a relatively low temperature, 700-800 degrees centigrade (Low Temperature), wherewith the ceramic mass is sintered and transformed to a ceramic. Subsequent to this curing or hardening process, it is usual to speak of layers instead of tapes.
In the case of applications for high frequency signals, particularly within the microwave field, it is not always possible to use traditional conductors, since this would result in unacceptable losses and disturbances. A normal requirement in the case of microwave signals is the presence of an earth plane above or beneath a conductor, this earth plane following the conductor. When a conductor only has an earth plane on one side it is called a microstrip. These strips are normally arranged so that they have the printed circuit board on one side and air or a corresponding dielectric on the other side. In other cases, it is desirable that the conductor is surrounded by both an upper and a lower earth plane, this conductor then being called a stripline. When the distances between a stripline and the earth planes are the same on both sides of the conductor, it is said that the stripline is symmetrical. When these distances are mutually different, the stripline is said to be asymmetrical.
Although symmetric striplines are the most common, there are occasions when an asymmetric stripline is preferred; for instance, the performance of a transition becomes poorer the greater the distance between the two striplines. Consequently, a transition between an asymmetric stripline close to an earth plane to an asymmetric stripline that is close to the other side of the earth plane may be more effective than a transition between two symmetric striplines. The transition between symmetric and asymmetric striplines is sometimes made between the same earth planes, which, in principle, is what is shown in FIG. 1.
One advantage afforded by striplines is that radiation from the conductors is small when, e.g., transmitting signals in the microwave range in so-called stripline-mode, which is one reason why such signals are often transmitted in this way. Microstrips and striplines can be easily provided in multilayer printed circuit boards, and are consequently often used to this end. In order to enable conductors to be surrounded by earth planes, conductor planes and earth planes are normally disposed alternately in the printed circuit board.
It is necessary to follow special construction rules in the production of multilayer printed circuit boards. For instance, in order to satisfy the rules a given smallest tape thickness is required for a smallest conductor width of a 50-ohm conductor. The rules also require the diameter of the vias to be in the same order of magnitude as the thickness of the tape. Consequently, in the case of the transition shown in FIG. 1, the diameter of the via is greater than the width of the asymmetric stripline. In order to compensate for this and to ensure only a small margin of error so that the via will safely meet the conductor, it is normal to place a cover pad on one end of the conductor; see FIG. 2. The presence of the cover pad results in a mismatch between the via and the conductor.
Mismatch problems do not actually occur in the case of low frequency signals. Mismatch, however, is a problem in the case of RF signals, for instance. Because it is otherwise usual to conduct RF signals in a so-called stripline mode, which reduces radiation from the conductors, it is desirable to solve problems associated with mismatching.
The present invention addresses the problem of improving matching in the transition between a symmetric and an asymmetric stripline.
Accordingly, one object of the present invention is to provide a transition between symmetric and asymmetric striplines that results in good matching.
In brief, the present invention provides an arrangement with a transition between a symmetric an asymmetric stripline in which the earth plane closest to the transition is moved away through the distance of one level in the proximity of the via.
The invention arrangement is characterised by the features set forth in the accompanying claim 1.
Advantageous embodiments of the inventive method will be evident from the dependent claims 2-8.
One advantage with this solution to the problem is that transitions between symmetric and asymmetric striplines can be made with better matching, meaning that the printed circuit board will perform better and have less losses.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawing.